1. Field of the Invention
The present invention relates to an information source encoding apparatus such as a television conference/telephone signal encoding apparatus (CODEC).
2. Description of the Prior Art
FIG. 1 shows the structure of an information source encoding apparatus (excluding a code assigning unit) based on the International Telegraph and Telephone Consultative Committee's (CCITT) Recommendation H.261 for an encoding system for Television Conference/Telephone. In the same Figure, a subtractor 1 generates a prediction error signal 102 by subtracting the original signal 100 of a current image frame from a prediction signal 101 output from an intra-loop filter 8. An orthogonal converter 2 executes orthogonal conversion (such as discrete cosine conversion) of prediction error signal 102 from subtractor 1 through a selector 9 in an inter-frame (INTER) mode or original signal 100 in an intra-frame (INTRA) mode, respectively, in units of pixel blocks in order to generate a conversion coefficient 103. A quantizer 3 quantizes, in the inter-frame mode, all elements of respective conversion coefficients 103 from orthogonal converter 2 in the same quantizing step width, while, in the intra-frame mode, DC and AC components of conversion coefficients 103 in the same or different quantizing step. The quantized result is outputted as a quantizing index 107 (binary encoded subscript numbers for discriminating a quantization representative level value). A dequantizer 4 and an inverse orthogonal converter 5 conduct, to quantizing index 107, an inverse process of the quantization and the orthogonal conversion to recover a conversion coefficient as a decoded prediction error signal 108. An adder 6 adds the decoded prediction error signal 108 and prediction signal 101 from intra-loop filter 8 through a selector 10 to generate a local decoded signal 109. A motion compensation predicting circuit 7 executes a pattern matching calculation in units of pixel blocks to local decoded signal 109 of a preceding image frame and original signal 100 of a current image frame from adder 6 to generate prediction signal 101 and a motion vector 110 For outputting an optimum prediction. Intra-loop filter 8 executes a filtering process of prediction signal 101 from motion compensation predicting circuit 7 to suppress accumulation of encoding noise. For example, the necessity of the filtering process is judged based on the amplitude of the motion vector and an intra-loop filter ON/OFF signal 111 is outputted. Selectors 9, and 10 respectively switch between original signal 100 and prediction error signal 102 and between an idle signal and prediction signal 101 with an intra/inter-frame mode signal 114 from an encoding controller 14. Encoding controller 14 is an external structural element of the information source encoding apparatus for adaptively controlling and discriminating between the intra/inter-frame modes, and outputs intra/inter-frame mode signal 114 and an intra/inter-frame discrimination flag 115. Moreover, controller 14 also controls the quantizing step width in response to the amount of generated information so as to output a quantization characteristic designation signal to quantizer 3.
The information source encoding apparatus of the prior art explained above employs a system to control the quantizing step width only to adapt tile amount of generated information (single adaptive control system).
FIG. 2 illustrates the situation that a coefficient block B to be quantized in the intra-frame mode exists within coefficient blocks (a quantizing unit of the orthogonal conversion coefficient regions bounded with dotted lines) to be quantized in the inter-frame mode. If a small amount of information in generated (movement of image is small), quantizer 3 quantizes all elements of coefficient blocks A in a fine quantizing step width, For example, in "level 4" depending on quantization characteristic designation signal 112 from encoding controller 14. Meanwhile, all elements of coefficient block B are quantized in a coarse quantizing step width, For example, in "level 8". It is a measure against quantization overload in the quantization of coefficient block B to also apply "level 8", which is the quantizing step width of a DC component based on the Recommendation H.261 of CCITT, to an AC component. If "level 4" is also applied to the quantizing step width of an AC component, as in the case of peripheral coefficient blocks A, quantization overload is inevitably generated, easily resulting in visible overload noise if the AC component is fully extended up to the limit of dynamic range which is specified as eleven bits by Recommendation H.261 of CCITT.
In such an information source encoding apparatus of the prior art, when a region for periodical encoding (the frequency is defined as "once per 132 frames" based on Recommendation H.261 of CCITT) in the intra-frame mode a region of encoding in the inter-frame mode coexist within a comparatively stationary image screen, the intra-frame mode encoding region having the coarse quantizing step width is included in the inter-frame mode encoding region having the fine quantizing step width so that overload noise cannot easily be visualized. Accordingly, block-type noise, including discontinuity, can be visualized in a recovered image at the boundary between an intra-frame encoding region including a large quantizing distortion and a peripheral inter-frame encoding region.